@FleetFlyer: Thank you, the joys of flying in France I have been beginning to discover. It is the UK that must wait.
@Peter: who mentioned NOT flying over water? I have crossed both the rivers Schelde and Meuse!
@GtW: I thought I had done that calculation before, but looked into it again. Surprise! Disregarding wind, and the height of the famous white cliffs of Dover, I calculated 35 kms to cross, so max 18 kms to glide. With a 9:1 glide ratio*, that would require 2000 m of altitude which comes to FL65 in ISA. The one question is whether I'd be allowed to go that high - microlights are normally not allowed in controlled airspace. But I don't yet have any UK maps or charts to check.
*from
EuroFOX Aviation Ltd - Aircraft spec and performance - the EuroFox is, ahem, how shall I say, err, closely related to my Apollo Fox. The same web page has some interesting weight figures.